The substitution of transparent components of mineral origin with transparent components made of moulded plastic material provides a functional alternative at low production cost, and complete independence from certain supply sources of up-market transparent materials.
However, moulded transparent parts are, on the one hand, relatively thick since it is difficult to mould very thin parts with adequate reproducible quality. On the other hand, the wear resistance, and particularly scratch resistance of these known plastic materials is very mediocre. The use thereof for the external components of timepieces—the middle part of the case, bezel or crystal—is thus limited in time.
This low wear resistance also makes it impossible to produce components with sharp angles, which would become blunt too quickly over time on contact with the user, his clothes, and ordinary everyday objects.
EP Patent Application No 0320811A1 in the name of ETA describes a method of making a solid mould for parts of very small dimensions, with shells derived from powder preforms transformed by sintering.
JP Patent Application No 58080587A in the name of SUWA SEIKOSHA describes an improvement to watch cases, in terms of resistance and dimensional precision, by the use of fibre glass treated with acrylic silane and mixed with a synthetic UV hardening resin, such as spirane resin, with a photo-polymerisation initiator agent such as benzophenone. These watch cases are moulded in elastic synthetic rubber or silicon moulds, which are fixed to glass plates and then UV-exposed for 30 seconds to 30 minutes depending on the intensity of radiation.
CH Patent Application No 369407A in the name of LONZA describes the employment of a mixture of polymethylmethacrylate and polyglycol dimethacrylate for manufacturing watch crystals or optical glasses.
EP Patent Application No 0549978A1 in the name of ETA describes a method of assembling a crystal on a watch case made of heat weldable thermoplastic material, with mechanical indexing between the crystal and the case.
US Patent Application No 2004/0241049A1 in the name of Bruce CARVALHO describes the employment of flexible elastomer tools for producing thermoplastic components.
WO Patent Application No 2012/025291A2 in the name of ETA SA describes the employment of silicon based polymers for making amorphous ceramics using a pyrolysis process.